In the past various attempts have been made to design hypodermic syringes with retractable needles. Typical examples of such devices are shown in U.S. Pat. Nos. 4,838,863; 5,019,044; 5,064,419; 4,950,241; and 4,978,343.
Some of these patents show the needle retracted into a hollow piston or barrel of a syringe either manually or by a spring which is biased to move the needle into a stored position either within a hollow piston or at least within the barrel of a hypodermic syringe. Such devices are only as effective and reliable as the design of the mechanisms used to retract the needle and some mechanisms may either fail to retract the needle completely or may fail to retain the needle in a retracted position.
The present invention is a further development of the concept shown in prior U.S. Pat. No. 5,180,370 issued to E. R. Gillespie which uses a hollow plunger in a medical syringe as a needle storage compartment when the needle has been retracted inside the syringe after an injection has been given. One primary advantage of the hollow plunger is that the needle can be in the stored or retracted position inside the plunger when the plunger is pressed into the barrel. In other patents which do not show a hollow plunger, the plunger must either be left protruding from the rear end of the barrel after needle retraction or in some instances the protruding portion of the plunger is broken off at the rear end of the barrel. Either option is not as desirable as having substantially all of the plunger contained inside the barrel after the needle has been retracted.
Both the prior Gillespie patent (U.S. Pat. No. 5,180,370) and the present invention provide a means of hydraulically sealing off the interior of the hollow plunger from the fluid chamber of the syringe.
The prior Gillespie patent mentioned above, uses an end cover member over the front end to the plunger to seal of the interior of the hollow plunger from the fluid chamber.
The present invention uses an elongated tube structure for sealing off the interior of the plunger from the fluid chamber and for receiving a bias spring for retracting a needle and needle mounting plug into the syringe.
Many additional patents have been issued on retractable needles since the prior Gillespie patent. Typical examples of such patents are U.S. Pat. No. 5,188,599 (Botich et al.); U.S. Pat. No. 5,190,526 (Murray et al); and three U.S. Pat. Nos. 5,267,961; 5,389,076; and 5,423,758 (all of which are issued to T. R. Shaw).